The present invention relates to transmission fiber-optic sensors, and more particularly to transmission fiber-optic temperature sensors utilizing two parallel fibers that approach a temperature sensitive material from the same direction.
Several types of fiber-optic transmission sensors for temperature measurement are known in the art. For these sensors the fibers are used to guide light to a temperature sensitive material and back to a detector. Examples of such prior art sensors are found in U.S. Pat. Nos. 4,376,890; 4,462,699; 4,223,226; 4,313,344 and British Patent No. UK 2025608. Another type of fiber-optic temperature sensor is disclosed in Kyuma et. at., IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982, pages 676-679. Kyuma discloses a fiber-optic instrument for temperature measurement that uses two light emitting diodes (LED's) as light sources. Each LED has a different wave length. Optical pulses from each of these LED's are guided through a fiber-optic channel that includes the fiber-optic sensor made from a semiconductor material. One LED is selected to emit light with a photon energy near the band gap energy of the semiconductor sample. The absorption of this light in the semiconductor sample is a function of the temperature. The second LED emits light with a photon energy less than the band gap of the semiconductor material, and is therefor not absorbed in the semiconductor sample. This second light source is used as a reference so that attenuation changes in the fiber can be eliminated from the temperature measurement.
Despite the fact that the above-described prior art fiber-optic temperature sensors generally use some sort of a reference signal in order to minimize or eliminate effects of fiber-optic attenuation, the resulting temperature measurements are nonetheless subject to variations in the light intensity originating at the source of light. Moreover, where the temperature sensitive element absorbe light falling withina prescribed frequency range, variations in the frequency of the input light source can also adversely affect the temperature measurement. Further, where two light sources are used, as is the case in Kyuma et. al., a change of the intensity ratio of the light generated by the two LED's can influence the temperature measuremen. A change of the intensity ratio can occur, for example, out of different aging properties associated with the LED's. Further, the temperature range that can be measured may be limited due to the particular frequency spectrum of the LED or other light source that is used.
A common problem associated with fiber-optic sensor applications is to measure the temperature in a very narrow cavity. This necessitates that the input and output fibers be parallel to each other at the entrance of the cavity. However, the operation volume at which the actual temperature sensitive material is located must be determined by the radius of the fiber loop because, as taught in the prior art, the input and output fibers must share a common axis. This operation volume is much larger than the volume of the fiber or of the temperature sensitive material. Hence, the fiber-optic sensors of the prior art are limited for use in an operation volume that is not less than the radius of a fiber loop.